Higher strength of steel material is much demanded in an environment marked by the call for improvement of automobile fuel economy and trimming of weight, and in the field of cold-rolled steel sheets, the tendency toward high-tensile strength steel sheets (hardness enhancement) is advancing. On the other hand, cold-rolled steel sheets are press-formed in the course of manufacturing components, but this can be possible on the premise that the steel sheets retain sufficient ductility such as elongation. While addition of alloy elements is effective for enhancement of strength, ductility tends to decrease as the added quantity of the alloy element increases.
Among the alloy elements as abovementioned, Si is an element that causes relatively small decrease in elongation and, therefore, is useful for achieving enhancement of strength while maintaining elongation. Increase in Si content, however, causes degradation in chemical conversion treatability resulting in inferior film adhesion after coating. For this reason, when the chemical conversion treatability was given more importance, the Si content was obliged to be decreased. Also, the cracks attributable to Si-containing grain-boundary oxide formed on the surface of the steel sheet in case the Si content increased became a factor in deterioration of coated film adhesion.
As the technology hitherto used to satisfy both of mechanical properties and chemical conversion treatability, there is a technique by which the steel sheet is covered in the surface with a clad member, thereby providing a low-density Si layer in the surface for high chemical conversion treatability and securing mechanical properties with a high-density Si layer on the inside (e.g., the patent document 1). The necessity of adopting a clad structure, however, entails the problem that the manufacturing process becomes complex resulting in increased manufacturing cost.
There is also another conventional technique in which a special alloy element is added to prevent Si, the harmful factor against chemical conversion treatability, from becoming concentrated in the surface (e.g., the patent documents 2 and 3). In this method, addition of Ni or Cu suppresses concentration of Si in the surface layer of the steel sheet securing chemical conversion treatability. However, this method has a problem in that the use of expensive Ni or Cu pushes up the cost.
The steel material used by these methods has a low C-content as below 0.005% and it relates to the so-called IF steel intended for enhanced deep drawing quality by controlling texture at a specific recrystallization temperature. With such IF steel sheets containing very low C-content, it is difficult to attain the level of high tensile strength as intended by the present invention.
The patent document 4 describes a case where the chemical conversion treatability is secured by using precipitated NbC as crystal nucleation sites for crystallization of zinc phosphate. This technique is also to secure the deep drawing quality by controlling texture in the low C-content region below 0.02%, but it is undeniable the steel sheet thus obtainable shows insufficiency in strength even though its C-content is somewhat higher than the above IF steels.
The patent document 5 proposes a retained-austenite containing steel sheet which secures chemical conversion treatability with a defined ratio of SiO2/Mn2SiO4 in the surface layer. Since this technique needs to control formation of oxide in the surface layer and elemental ratio of Si/Fe, it is necessary either to remove the Si oxide formed on the surface after continuous annealing by means of acid pickling or brushing, or to suppress the forming volume of Si oxide by regulating the dewpoint at over −30° C. at a temperature above Ac-1 transformation point.
However, the treatment by acid pickling or brushing requires increased manufacturing steps incurring a rise in manufacturing cost. Also, as far as the embodiment shown in the document indicates, even if the dew point control which is carried out inside the continuous annealing furnace is exercised, the best available result will be about 1.0 for the ratio of SiO2/Mn2SiO4 in the uppermost layer, and further, the chemical conversion treatability cannot be said to have been sufficiently improved inasmuch as SiO2 that will disturb formation of chemical conversion film crystal will be produced in an amount roughly equal to Mn2SiO4.
The patent document 6 proposes the technique that by observing the surface of the steel sheet with XPS (X-ray photoelectron spectroscopy), the ratio of Si, from which oxide is composed, against Mn (Si/Mn) should be constricted below 1 thereby enhancing the chemical conversion treatability.
It is a common knowledge that the steel having Si/Mn ratio of 1 or below, such as the mild steel in which the Si content is almost zero or the steel sheet having a Si content of 0.1% or below, has a good chemical conversion treatability. However, as above-mentioned, it is necessary that the steel sheet should have a certain extent of Si content in order to improve both strength and ductility, and yet there is a limitation in decreasing Si content to make Si/Mn ratio 1 or below. Even if Si/Mn ratio could be kept 1 or be low by controlling Mn quantity to an appropriate level while securing proper amount of Si, it would not necessarily ensure that a steel sheet provided with good chemical conversion treatability could be stably obtained.
Incidentally, known as a steel sheet that can enhance both strength and ductility at a time is the retained austenite steel; retained austenite (γR) produced in its constitution causes induced transformation (strain induced transformation or TRIP=transformation induced plasticity) during work deformation and thereby enhances ductility. As commonly used methods to keep such retained austenite subsisting stably under room temperature, there are two methods; one is to make it contain about 1-2% of Si, and the other is to make it contain about 1-2% of Al in place of Si.
The above method of making Si positively contained can enhance both strength and ductility at a time, but the method is apt to form Si-based oxidative film on the surface of the steel sheet, because of which the chemical conversion treatability of the sheet becomes inferior. On the other hand, the method of making Al positively contained can yield a steel sheet of comparatively good chemical conversion treatability, but in point of strength and ductility, this steel sheet is inferior to the aforesaid Si-containing steel sheet. Since Al is not an element having intensifying functionality, addition of C, Mn, and other elements of intensifying power in lavish doses is necessary in order to obtain enhanced strength, even though such measure again entails deterioration in weldability, etc.
From the viewpoint of improving mechanical properties, it is also proposed to positively add both Si and Al in the retained austenite steel sheet (e.g., the patent document 7). Still another proposal suggests a steel sheet in which improvement is made of stretch flangeability, in which performance the retained austenite containing steel sheet has shortcomings (e.g., the patent document 8). These steel sheets are also apt to form Si-based oxidative film on the surface owing to lavishly added Si and are thus perceived to be subject to inferior performance in point of the chemical conversion treatability of the sheet. These sheets are neither improved with respect to hydrogen embrittlement resistivity which is commonly regarded as a drawback of the retained austenite steel sheet.    [Patent Document 1] JP-A-5-78752    [Patent Document 2] Japanese Patent No. 2951480    [Patent Document 3] Japanese Patent No. 3266328    [Patent Document 4] Japanese Patent No. 3049147    [Patent Document 5] JP-A-2003-201538    [Patent Document 6] JP-A-4-276060    [Patent Document 7] JP-A-5-117761    [Patent Document 8] JP-A-2004-238679